Shades of gray on computer printers are achieved by a process called halftoning. In the halftoning process, halftone cells are created each including a number of pixels. By making all the pixels white, a white spot is created and, conversely, by making all the pixels black, a black spot is created. By making only a small percentage of the pixels black in a cell, the spot, to the eye, appears light gray and conversely by making a large percentage of the pixels black in a cell, the spot appears dark gray. In general, the number of gray levels available is a function of the number of pixels in a halftone cell.
However, a given printer's capacity is a function of the number of pixels per inch and the size of the halftone cell. For example, if a cell includes 9 pixels, there are 10 gray levels possible while if the cell includes 256 pixels, there are 257 possible gray levels. Thus, the finer the halftone screen, the fewer number of pixels in each cell and therefore the fewer gray levels available. So, a 600 dpi printer at a screen frequency of 200 lines per inch (lpi) has 10 possible gray levels while the same printer at a screen frequency of 120 lpi has 26 possible gray levels. Therefore, to achieve more gray levels, screen frequency and hence image resolution suffers.
So, the goal has been to increase the printer's image resolution on 600 dpi printers and achieve more gray levels at the same screen frequency. That is, a 600 dpi printer at 141 lpi results in 19 gray levels but with super dot techniques can produce 145 gray levels at 141 Ipi.
Even with such advancements in technology and the use of screen angling and spot spacing variation techniques, however, a phenomenon known as mottling occurs which makes photographic and other images appear to have bad complexions. See, in general, Real World Scanning and Halftones, Blatner and Roth (1993, Peachpit Press).
Current processes for correcting the mottling effect include techniques that unfortunately add significant storage requirements to the printer system.